This invention relates to a process for upgrading heavy hydrocarbon oils.
There is a growing need for new methods of exploiting Canada's remaining energy resources in the most economical and efficient way. In particular, because of vast existing resources in the form of heavy hydrocarbon oils, e.g. heavy oils and tar sands bitumens, there is an urgent need to develop more efficient means for upgrading these heavy feedstocks.
One technology that has been investigated in recent years is thermal pyrolysis of the heavy hydrocarbons. One such pyrolysis technique is known as ultrapyrolysis. In ultrapyrolysis, reactants are heated at maximum rates in the order of 10.sup.5 .degree. K/s to temperatures in the range of 600.degree. to 1000.degree. C. with resident times of less than 950 milliseconds. Ultrapyrolysis of heavy oils has proved to be an effective method by which these hydrocarbons can be upgraded to low viscosity fluids and valuable products, such as olefins, while greatly reducing the undesirable coke deposition.
Spouted beds have been recognized for many years as a distinct method of contacting solids and fluids. In the spouted bed, fluid is introduced at a single orifice in the bottom of a vessel containing large granular solids. If the bed height is below a certain maximum value, the fluid clears a passage up through the bed and the bed is said to be "spouting". In passing through the spouted bed, the fluid entrains granular solid material which is carried up the spout with it. On emerging from the top of the bed, the fluid jet diverges and loses momentum and its capacity for transporting the entrained solids is considerably reduced. These solids thus rain out and settle on top of the annular bed around the spout, which continues to exist where the original bed was.
Particularly for achieving conditions for ultrapyrolysis in a spouted bed, a draft tube may be included. The draft tube is a tube located within the bed such that it is actually aligned with the spouting fluid inlet. It is located some distance up from the inlet and extends out of the top of the bed. The distance between the spout inlet and the draft tube is termed the entrainment section since this is the area within the bed where there is a transfer of solids from the annular region surrounding the draft tube to the spout.
In a conventional spouted bed, the spout is in continual contact with the annular solids over the entire height of the bed and so to exist, the spout jet must be able to support the surrounding annular bed. In order to provide this support, the spout must be of greater pressure than the annulus. Because of this pressure difference, there is a continual percolation of spouting gas into the annulus. If the bed is too deep, the spout cannot support the bed and collapses. The bed height at which the transition from spouting to collapse occurs is the maximum spoutable bed height.
The inclusion of a draft tube removes the bed height limitation. By physically separating the spout from the annulus, it provides support for the annular solids. More importantly, the draft tube also provides or, at least, reduces the loss of spout gas and the entrained solids act as the heat carrier. Because of the high differential velocity between the solids as they are entrained and the entraining gas, the transfer of heat from the solids to the carrying gas is very high. Having confined the reactant, the draft tube essentially becomes the tubular reactor.
Bartholic, U.S. Pat. No. 4,446,009 describes a selective vaporization process for decarbonizing and demetallizing heavy hydrocarbon fractions in a riser column. That system uses a temperature of about 500.degree. C. and a residence time of about 2 seconds.
Circulating fluidized bed systems are also widely described in the literature, for example in Voorhies et al, U.S. Pat. No. 2,849,384, which shows a fluid coking system which uses a draft tube with recirculation of solid particles. However, such systems are not designed for ultrapyrolysis.
A major problem with most systems used for upgrading heavy oils is the large amounts of coke produced because of the long contact times. For instance, conventional pyrolysis upgrading reactors available commercially, such as fluid bed cokers, may lay down as much as 20% coke, while delayed cokers may yield up to 30% coke from heavy oils.
It is the object of the present invention to provide an improved internally circulating fluidized bed reaction process and system for ultrapyrolysis of heavy hydrocarbon oil.
It is a further object of the invention to be able to upgrade heat oils in a low pressure reactor (e.g. a pressure of less than 20 atmospheres) under conditions where coke yields are significantly less than 10%.